On the triggering of volcanic eruptions by Earth tides

Abstract

erupted at or near the fortnightly tidal maximum is located in an area having a negative Bouguer anomaly, a large crustal thickness, and a small rate of horizontal crustal deformation. Conversely, volcanoes in areas characterized by thinner crusts and crustal deformation rates greater than 3.0 cm/yr generally erupt at or near the fortnightly tidal maximum. Solar and lunar tides produce in the earth's crust quasi-periodic stresses with amplitudes less than 10 s N/m s (10 -3 bar), a maximum rate of change of I N/m s sec (10 -5 bar/sec), and a maximum spatial gradient of 10 -s N/m s (10 -s bar/m). This is the largest short-period oscillatory stress in the earth's crust, and suggestions that it plays a role in triggering slowly accumulating tectonic stress have been frequent [Morgan et al., 1961; Knopoff , 1964; Stacey, 1964; Simpson, 1967;Shlien, 1972]. Detailed investigations, however, have been focused almost entirely on the question of triggering earthquakes by earth tides. It is now generally accepted that earthquakes and aftershock sequences occur at times uncorrelated with earth tides, although some particular aftershock sequences appear to be an exception [Ryall et al., 1968]. A second source of tectonic stress variation likely to be affected by periodic tidal stress occurs with volcanic eruptions. In contrast to the earthquake problem, where the rate. of stress accumulation and the geographic location of